Furans ring cleavage

A remarkable formation of difurotropylium salt 246 (Scheme 60) proceeds by hydride abstraction from 244, furan ring cleavage, and recyclization or similarly via intermediate 245 (92KGS1142). 

Analysis of excreta samples indicated that both psoralen and Isopsoralen are metabolized extensively by polyxenes caterpillars, primarily by the same furan ring cleavage reactions observed in our earlier studies with xanthotoxin (Figure 3). 

Scheme 7 Transformations of HMF involving furan ring cleavage...

The reaction with aniline salts was explained simultaneously by Zincke and by Dieckmann in 1905. The furane ring undergoes amino-lytic cleavage and then the anil is formed from the aldehyde. 

The five-membered ring heteroaromatics (furan, thiophene, and pyrrole) show very similar ring cleavage patterns. The first step in each case is cleavage of the carbon-heteroatom bond. 

Catalytic reduction of pyrroles gives successively A3-pyrroles and pyrrolidines. Tetrahydrofurans are formed by the catalytic reduction of furans with Raney nickel and hydrogen ring cleavage products... 

Oxidative ring fission of furans using the commercially available reagent pyridinium chlorochromate (PCC) has been studied as well (80T661). Experimental evidence supports the preliminary formation of intermediate (87) formed by 1,4-electrophilic attack of chlorochromate anion upon the furan ring. This intermediate then breaks down by heterolytic cleavage of the Cr—O bonds to afford initially the cis enedione which isomerizes to the trans product. Treatment of (88) with sodium hydroxide in methanol effects ring closure with formation of the 4-methoxycyclopentenone (89 Scheme 22). 

Pyridinium chlorochromate in dichloromethane reacts with furans to give a range of products, but they are all formed by 1,4-electrophilic attack of chlorochromate on the furan ring the fate of the resultant intermediate (57) by heterolytic cleavage of the Cr—O bond depends on the substituents at the a-positions of the substrate. 2,5-Dialkylfurans yield a, /8-unsaturated-y-dicarbonyl compounds, 5-methyl-2-furylcarbinols yield pyran derivatives, and 5-bromo-2-furylcarbinols yield 5-hydroxyfuran-2(5//)-ones (Scheme 25) (80T661). 

Ring cleavage and decarbonylation has also been observed in benzo[h]-furan-2-one.170 Careful examination of the 3-phenyl derivative has shown that decarbonylation arises by excitation of the keto tautomer (210) and yields the o-quinone methide (211)171 this is readily trapped as the ether... 

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